var a = new Promise(function (resolve) {
  setTimeout(() => {
    resolve('I ❤ U')
  }, 10);
}).then(res => {
  return new Promise(function (resolve) {
    setTimeout(() => {
      resolve('lagou ' + res)
    }, 10);
  })
}).then(res => {
  return new Promise(function (resolve) {
    setTimeout(() => {
      resolve('hello ' + res)
    }, 10);
  })
}).then(res => {
  console.log(res)
})